CN1663324A - Electronic dimming ballast for compact fluorescent lamps - Google Patents

Abstract

The operating frequency of a ballast is sufficiently far from resonance of a resonant tank circuit (430) that, when a compact fluorescent lamp (500) is dimmed to about one percent light output level, the ballast is operating with an open loop gain below a predetermined level, and is operating with an output impedance greater than a predetermined and more preferably, greater than twice the absolute value of the maximum negative incremental impedance of the lamp. By changing the frequency to 85 kHz from 80 kHz through control circuit (418), for example, the gain is maintained about the same whether the light output is at five percent or one percent, for example.

Description

Electronic dimming ballasts for compact fluorescent lamps

technical field

This invention relates generally to dimming gas discharge lamps and, in particular, to electronic dimming ballasts for dimming compact fluorescent lamps.

Background technique

Gas discharge lamps efficiently convert electrical energy into visible light. A gas discharge lamp is usually a long gas-filled (typically low pressure mercury vapor) tube with electrodes at each end. Each electrode is typically formed from a resistive wire (typically tungsten) coated with a thermionic emissive material such as a mixture of basic metal oxides.

The steady state operation of a typical gas discharge lamp is as follows. Applying a voltage across the resistive wire heats the electrodes to a temperature sufficient to cause thermionic emission of electrons into the discharge tube. A voltage applied between the two electrodes accelerates the electrons towards the anode. On the way to the anode, electrons collide with gas atoms to produce positive ions and additional electrons, forming a positive gas plasma and negative charge carriers in the tube. Electrons continue to rush to the anode and positive ions to the cathode, continuing the discharge in the tube and further heating the electrodes. If the applied power is AC, the electrodes reverse polarity every half cycle.

The discharge causes the emission of radiation at a wavelength that depends on the specific gas fill and the electrical parameters of the discharge. Because each collision creates additional electrons and ions, the increase in arc current causes a decrease in the impedance of the lamp, typically referred to as "negative incremental impedance." Due to the nature of this negative incremental impedance, operation of the lamp is inherently unstable, therefore, the current flow between the electrodes must be controlled to provide stable lamp operation.

Gas discharge lamps, including fluorescent lamps, are designed to deliver their full, or "nominal", light output at a specified value of RMS lamp current. In the specification and appended claims, the full light output of a lamp is referred to as "nominal light output".

Fluorescent gas discharge lamps include a phosphorous coating on the inner surface of the glass tube envelope and provide visible light output by exciting the coating with radiation from the discharge. Conventional fluorescent lamps are generally long straight tubes of substantially circular cross-section with varying diameters ranging between about 5/8 and 1.5 inches.

Compact fluorescent lamps differ from conventional fluorescent lamps in that they are constructed from smaller diameter tubes, typically less than about 5/8 inch outside diameter. Additionally, the light is small in part because the tube has one or more small radius bends so that the tube folds back on itself to achieve the compact shape. Also, in compact fluorescent lamps, where the tube itself is folded back, the ends of the lamp are generally very close to each other.

Referring to FIG. 1 , a prior art lamp system 10 includes, for example, a 120V 60Hz AC power supply. Sinusoidal voltage 100 , phase control dimmer 102 , electronically dimmable fluorescent ballast 200 and compact fluorescent lamp 300 .

Ballast 200 receives input power (or hot) on line 202 , a variable input dimming signal (or dimmed hot) on line 204 , and neutral on line 206 . It should be understood that the voltage on line 202 is rectified by full wave bridge rectifier 209 within ballast 200 to produce a voltage that has a positive DC average value relative to normal circuits.

Electronic dimming ballast 200 is designed to provide an amount of output power to lamp 300 based on a variable input signal on line 204 from dimmer 102 . Dimmer 102 is a phase control dimmer that provides a variable input signal on line 204 by varying its phase firing angle, which controls the RMS value of the variable input signal on line 204 .

As is known in the art, ballast 200 generally includes a first power stage having a boost circuit 210 that receives rectified voltage from rectifier 209 and produces a high voltage on line 214 that can reach 400 VDC or higher DC.

The ballast 200 also typically includes a second power stage having an inverter circuit 216 that converts the DC voltage on line 214 into a high frequency switched voltage that is applied to a resonant circuit (resonant circuit 216). tank circuit) 230, the resonant circuit 230 provides a suitable AC voltage to drive the lamp 300. A high voltage energy storage capacitor 212 is disposed in a shunt configuration opposite line 214 to provide a low impedance current source to inverter circuit 216 .

The control circuit 220 provides control signals to the boost circuit 210 and the inverter circuit 216 via lines 221 and 222, respectively. The control circuit 220 controls the boost circuit 210 to provide a desired DC bus voltage, and controls the inverter circuit 216 to provide a high frequency converted voltage to the resonant circuit 230 . As a result, the ballast provides the desired current and voltage to lamp 300 via line 208 in response to the variable voltage input signal on line 204 so that lamp 300 illuminates at the proper brightness.

Control circuit 220 generally controls inverter 216, for example by comparing the rectified version of the variable input signal on line 204 with a signal representative of the current delivered to the lamp via line 208, and adjusting (via known error signal techniques) Line 222 inputs a control signal to inverter 216 to control the appropriate current to lamp 300 .

Control circuit 220 also controls boost circuit 210 to generate the appropriate DC output voltage on line 214 as is known in the art. Moreover, the control circuit 220 generally includes circuits for implementing other functions such as low-voltage lockout, over-current protection, over-voltage protection, and the like.

In the embodiment shown in FIG. 1 , control circuit 220 , boost circuit 210 and inverter circuit 216 are powered by control circuit power supply 240 . It should be understood that the control circuit power supply 240 can be implemented using any number of circuit configurations.

The lamp system 10 of Figure 1 requires three wires between the dimmer 102 and the ballast 200, which itself may be located in the light assembly. The system has been developed so that no third terminal is required on ballast 200 to receive the variable input signal on line 204 . In these systems, a variable input signal is received on line 202 . Other systems have been developed to utilize third and fourth terminals on the ballast 200 to receive variable input signals.

Typically, when dimming a linear fluorescent lamp down to a low level of light output (eg, around 1% light output level), it is necessary to increase the output impedance of the electronic dimming ballast to maintain stable lamp operation and prevent visible flicker. Generally, the output impedance of the ballast is increased by driving the ballast at an operating frequency close to the no-load resonant frequency of the resonant circuit. The apparatus and method required to obtain high ballast output impedance are taught in U.S. Patent No. B1 5,041,763, the entire contents of which are incorporated herein by reference.

Furthermore, the inventors have discovered that compact fluorescent lamps, compared to typical linear fluorescent lamps, have an additional region of lamp instability at low levels of lamp current of about 1% of nominal light output. This additional region of instability manifests itself as a tendency for the lamp's light output to extinguish, or "drop out," as opposed to the flickering between various low light levels observed in linear fluorescent lamps. Although this phenomenon is not fully understood, it is believed to be related to the physical characteristics of the compact fluorescent lamp, such as the small tube diameter and the small radius and number of radii the lamp bends.

Accordingly, there is a need in the art for a ballast circuit that maintains a stable, flicker-free dimming range down to approximately 1% of the full light output of a compact fluorescent lamp.

Contents of the invention

In order to overcome the deficiencies of prior art ballast circuits, the present invention provides a dimming system and method for compact fluorescent lamps, including a ballast of this type having an inverter circuit having An operating frequency that drives a resonant output circuit having a predetermined no-load resonant frequency. The operating frequency of the inverter circuit is selected such that the open-loop system gain is below a first predetermined level (e.g., below 15) and the ballast output impedance is above a second predetermined level (e.g., at approximately the maximum negative incremental lamp impedance twice the absolute value).

According to aspects of the invention, the operating frequency of the inverter circuit is determined by a control circuit comprising a frequency determining resistor-capacitor (RC) network. When the lamp output is less than about 1% of the nominal light output, the component values of the RC network are selected such that the operating frequency of the inverter circuit is a predetermined function of the resonant frequency of the unloaded resonant circuit.

For the purposes of this document and the appended claims, the term "DC" means that the voltage or current waveform is unidirectional and may be pulsating regularly or pulsating irregularly. The term "AC" refers to a voltage or current waveform that reverses polarity and has alternating positive and negative values at regular recurring intervals. The term "DC component" refers to the average value of an AC or DC waveform. The term "AC component" refers to the portion of an AC or DC waveform remaining after subtracting its DC component.

The foregoing and other aspects of the invention will become apparent from the following detailed description of the invention if taken in conjunction with the accompanying drawings.

Description of drawings

For purposes of explaining the invention, the presently preferred embodiments are shown in the drawings, however, it should be understood that the invention is not limited to the particular methods and instrumentalities disclosed. In the attached picture:

Figure 1 is a high level block diagram of a prior art fluorescent lamp system;

Figure 2 is the voltage/current characteristic curve of a general fluorescent lamp;

Figure 3A is an exemplary diagram showing a V-I curve and a negative incremental impedance curve of a lamp according to the present invention;

3B is a partially enlarged view of the V-I characteristic diagram of the general compact fluorescent lamp shown in FIG. 3A at a very low lamp current level;

FIG. 4 is a graph representing gain versus frequency for describing an exemplary system according to the present invention;

Figure 5 is a high level block diagram of an example system according to the present invention;

Figure 6 is a schematic diagram of a portion of the control circuit of Figure 5; and

FIG. 7 is a schematic diagram of another portion of the control circuit of FIG. 5 .

Detailed ways

The light output stability of a lamp generally relates to the quality of the current source used to operate the lamp. Current source quality is described numerically by a quantity called output impedance. Output impedance is defined as the ratio of the change in RMS output voltage divided by the corresponding change in RMS output current in ohms. Thus, a current source that exhibits a change in current level of 0.001 amps resulting in a change in output voltage of 1 volt has an output impedance of 1 volt divided by 0.001 amps or 1,000 ohms.

Fig. 2 is a voltage/current characteristic diagram of a general fluorescent lamp. The incremental impedance of the lamp at any operating point on this curve is defined as the slope of the curve at that point. As can be seen from the figure, the lamp incremental impedance is positive at very low currents, becomes zero at the point of maximum voltage, and quickly becomes negative as the current continues to increase. It is obvious that there is a point at which the lighting incremental impedance achieves its most negative value, which is marked A in FIG. 2 . The point of maximum negative incremental impedance is the operating point at which the lamp is most unstable and most likely to exhibit variations in arc current and light output. Therefore, measurements of the current output impedance should be made at the point of maximum negative incremental impedance of the lamp in order to stabilize correct lamp operation.

Graphs of fluorescent lamp V-I curves and lamp negative incremental impedance for exemplary lamps in accordance with the present invention are shown in FIGS. 3A and 3B . For compact fluorescent lamps operating at less than about 1% of nominal light output, not only is instability observed at the peak of the negative step-up impedance, but instability in the lamp current level is also expected to be observed at the peak of the V-I curve.

As shown in more detail in Figure 3B, the V-I curve of a typical compact fluorescent lamp shows a steep slope of a "cliff" for which the lamp voltage drops rapidly from the peak of the curve to zero when the lamp is dimmed to about 1% Below the rated output of , the lamp current decreases incrementally very little. In other words, when attempting to reduce the lamp current to a level corresponding to a light output below about 1% of the nominal light output, the lamp tends to "flame out", ie, go out. Therefore, it is desirable to reduce lamp current levels without "falling off the cliff" as much as possible, i.e., operating in the region of the steep positive slope below the peak, as this is the region where the lamp is most sensitive to system disturbances causing "flame out" and lamp flickering . As described in detail below, the present invention provides an apparatus and method for operating a compact fluorescent lamp at low current levels without "cliff lowering", ie, without lamp extinction and flickering.

FIG. 4 shows a graph of ballast system open loop gain versus frequency, which is used to describe example systems and methods in accordance with the present invention. To improve stability, ballasts are traditionally operated as close to resonance as possible. Thus, when dimming a linear fluorescent lamp down to a low level of light output (eg, less than about 1% of nominal light output), incrementally increasing the output impedance of the ballast is required to maintain stable lamp operation. Generally, the ballast output impedance is increased by driving the ballast's operating frequency as close to resonance as possible.

As shown in FIG. 4, the no-load resonant frequency of the resonant circuit of the exemplary ballast is about 80 kHz. However, for compact fluorescent lamps, at the operating frequency of the ballast, the open-loop gain of the ballast is greatly increased when the fluorescent lamp is turned down to a low level of light output. For example, with one such ballast and compact fluorescent lamp, the inventors have found that below about 4% of the nominal light output, the system gain begins to rise rapidly due to the reduced loading of the compact fluorescent lamp on the resonant circuit. High gain makes it difficult to form a stable closed-loop controller. For example, in FIG. 4, the gain at a nominal light output of 1% is very high relative to the gain at a nominal light output of 5% at the resonant frequency. As a result the lamp output current becomes very sensitive to even the slightest fluctuations.

Consequently, compact fluorescent lamps have a tendency to burn out as described above, and are therefore more difficult to maintain operating at low light output levels than linear fluorescent lamps.

In accordance with the present invention, in order to reduce the gain, the operating frequency of the ballast is chosen to be completely off-resonant so that when operating the lamp at a nominal light output level of about 1%, the ballast operates with an open loop gain of, for example, 15 below a predetermined level . By varying the frequency eg from 80kHz to 85kHz the gain is approximately the same whether the light output is eg 5% or 1%. This provides a stable control loop.

But the operating frequency need not be raised so high that the output impedance of the ballast drops below a predetermined level (ie, the output impedance of the ballast should remain above a predetermined output impedance, eg the absolute value of the maximum negative incremental lamp impedance). By operating in this frequency range, compact fluorescent lamps can be dimmed to less than about 1% of light output without blowing out and without flickering.

Preferably, the exemplary ballast includes means for selecting the frequency of operation such that the open-loop system gain is below a first predetermined level (about 15) and the ballast output impedance is at a second predetermined level (about the maximum negative incremental lamp twice the absolute value of the impedance, but at least greater than the absolute value of the lamp's maximum negative incremental impedance), where this exemplary ballast is used to dim a compact fluorescent lamp below about 1% of its light output, an exemplary ballast is An exemplary ballast of this type includes a resonant output circuit having a predetermined no-load resonant frequency. The means for selection preferably comprises an oscillator with a frequency-determining RC network and component values selected so that the light output of the lamp at or below about 1% of the nominal light output is an unloaded resonant tank A predetermined function of the resonant frequency.

Open-loop system gain is defined as the ratio of the resonant circuit output voltage to the resonant circuit input voltage.

Referring now to the drawings of exemplary embodiments, wherein like reference numerals refer to like elements, FIG. 5 is a high level block diagram of an exemplary system in accordance with the present invention. The system provides power variations from sinusoidal power to compact fluorescent lamps 500. In FIG. 5 , the live and neutral inputs are provided to the front end 401 . The system generally includes a front end 401 that converts the AC input voltage from the power supply to a DC bus voltage stored on a bus capacitor Cbus in known manner.

The output of the front end 401 is supplied to the inverter circuit 416 via the capacitor Cbus, and the inverter circuit 416 supplies the high-frequency converted AC voltage to the resonant circuit 430 . More specifically, the inverter 416 is oscillator controlled and switches in a known manner with complementary operating cycles of the form D/1-D, where D is the operating cycle, thus driving the resonance with a pulse width modulated waveform circuit 430.

The resonance circuit 430 may include a DC blocking capacitor Cblock, a resonance inductor Ltank, and a resonance capacitor Ctank. Resonant circuit 430 converts the pulse width modulated waveform from inverter circuit 416 into a high frequency AC voltage for driving lamp 500 . The control circuit 418 compares the input representing the current through the lamp with the input dimming control signal representing the desired light output level to control the current through the lamp by adjusting the operating frequency of the inverter 416 and the load frequency. Current from resonant circuit 430 is provided to lamp 500 to achieve and maintain a stable discharge over a range of selectable power levels. The control circuit 418 is described further below.

A portion of the control circuit 418 including the current feedback circuit is shown in FIG. 6 . Operational amplifier U7: A is configured as an integrator that integrates the difference between the desired light level signal from the phase-to-DC circuit 602 and the signal from the lamp current detection circuit 604 . Phase-to-DC circuit 602 accepts the input control signal from the control input and converts it to a DC level representative of the desired light level output. Lamp current sensing circuit 604 provides a signal representative of the lamp current in a known manner. Comparator U3:B compares the output of the integrator with the output from oscillator 702 (FIG. 7) to generate the pulse width modulated waveforms that drive the switches in half-bridge inverter circuit 416 of FIG.

FIG. 7 shows another portion of the control circuit 418 of FIG. 5, including an oscillator 702 that controls the operating frequency of the ballast. Oscillator 702 includes part of integrated circuit U6, resistors R95 and R124, and capacitor C84. The frequency of the oscillator for low light output levels depends on the value of resistors R95 and R124 and capacitor C84. Capacitor C84 is charged through resistors R95 and R124. When it reaches a value determined by integrated circuit U6, integrated circuit U6 discharges capacitor C84.

The control circuit 418 also includes a frequency shift circuit 704 that changes the operating frequency of the oscillator by changing the charging current available from resistors R95 and R124 to C84 at higher desired light output levels.

Phase-to-DC circuit 602 supplies a desired light level signal, which is a voltage proportional to the dimmer control input, to resistor R28. When the voltage applied to the non-inverting input of U2:D through the voltage divider formed by resistors R5 and R28 is lower than the voltage applied to the inverting input of U2:D by the voltage divider of resistors R3 and R64 (near the dimming lower end of the range), the output of U2:D is low, and transistor Q1 is off. The operating frequency is then determined by the values of resistors R95 and R124 and capacitor C84. When the desired light level signal from the phase-to-DC circuit 602 increases, the voltage applied to the non-inverting input of U2:D increases, causing the output of U2:D to increase, thus turning on transistor Q1 so that the oscillator's The output current is proportional to the desired light level signal. The current output of the oscillator reduces the operating frequency of the ballast for ballasts operating at high light output levels.

Although the invention has been described for compact fluorescent lamps, the circuits described herein can control any type of gas discharge lamp. As certain changes could be made in the circuits described above without departing from the scope of the invention, it is intended that any matter contained in the above description or shown in the accompanying drawings shall be interpreted in a illustrative and not in a limiting sense.

The present invention can be embodied in the form of suitable computer software, or in the form of suitable hardware or a combination of suitable hardware and software, without departing from the spirit and scope of the present invention. Further details regarding such hardware and/or software will be apparent to those skilled in the art. Accordingly, no further description of such hardware and/or software is necessary here.

Although illustrated and described herein with reference to certain specific embodiments, the invention is not limited thereto. Rather, various modifications may be made within the spirit of the invention and the scope of equivalents of the claims without departing from the invention.

Claims (18)

1. An electronic dimming ballast for dimming a compact fluorescent lamp with maximum negative incremental impedance, said electronic dimming ballast having open-loop system gain and output impedance, said electronic dimming ballast Devices include: an inverter, having an output and an operating frequency; a resonant circuit, connected to the output of the inverter; and a control circuit that controls the operating frequency of the inverter such that the open loop system gain is less than a first predetermined level and the ballast output impedance is greater than a second predetermined level such that the ballast is capable of operating below about 1% of a nominal light output The compact fluorescent lamp operates with no visible flicker. 2. The electronic dimming ballast of claim 1, wherein the first predetermined level is about 15. 3. The electronic dimming ballast of claim 1, wherein the first predetermined level is determined to be approximately equal to the open loop system gain at approximately 5% of the nominal light output. 4. The electronic dimming ballast of claim 1, wherein the second predetermined level is approximately equal to twice the absolute value of the maximum negative incremental lamp impedance. 5. The electronic dimming ballast of claim 1, wherein the second predetermined level is approximately equal to the absolute value of the maximum negative incremental lamp impedance. 6. The electronic dimming ballast of claim 1, wherein the control circuit includes an oscillator including a frequency-determining resistor-capacitor (RC) network for determining the operating frequency. 7. An electronic dimming ballast as claimed in claim 6, wherein the RC network has component values such that the operating frequency of the inverter is a function of the no-load resonant frequency of the resonant circuit. 8. The electronic dimming ballast as claimed in claim 1, wherein when the operating frequency of the inverter deviates from the no-load resonant frequency of the resonant circuit, the gain of the open-loop system decreases. 9. The electronic dimming ballast of claim 1, wherein the operating frequency of the inverter is different from the no-load resonant frequency of the resonant circuit such that an open loop system at approximately 1% of the nominal light output The gain is approximately equal to the open loop system gain at about 5% of the nominal light output, and the ballast output impedance is greater than the absolute value of the lamp's maximum negative incremental impedance. 10. A method for dimming a compact fluorescent lamp by dimming a compact fluorescent lamp using an electronic dimming ballast of the type comprising a resonant circuit having an unloaded resonant frequency and a ballast output impedance Fluorescent lamps have a negative incremental impedance up to approximately 1% of their nominal light output, the method comprising: determining a predetermined maximum open-loop system gain; determining a predetermined minimum ballast output impedance; and selecting the operating frequency of the inverter such that the open loop system gain is below approximately said predetermined maximum open loop system gain and such that the ballast output impedance is above approximately said predetermined minimum ballast output impedance, whereby The compact fluorescent lamp was operated below about 1% of nominal light output with no visible flicker. 11. The method of claim 10, wherein the predetermined maximum open loop system gain is 15. 12. The method of claim 10, wherein the predetermined maximum open loop system gain is approximately equal to the open loop system gain at about 5% of the nominal light output. 13. The method of claim 10, wherein the predetermined minimum ballast output impedance is approximately equal to twice the absolute value of the maximum negative incremental lamp impedance. 14. The method of claim 10, wherein the predetermined minimum ballast output impedance is approximately equal to the absolute value of the maximum negative incremental lamp impedance. 15. The method of claim 10, wherein the step of selecting the operating frequency of the inverter includes providing a control circuit having an oscillator with a frequency-determining resistor-capacitor (RC) network. 16. The method of claim 15, wherein the RC network has component values such that the operating frequency of the inverter is a function of the no-load resonant frequency of the resonant circuit. 17. The method of claim 10, wherein when the operating frequency of the inverter deviates from the no-load resonant frequency of the resonant circuit, the open-loop system gain decreases. 18. The method of claim 10, wherein the operating frequency of the inverter is different from the no-load resonant frequency of the resonant circuit such that the open-loop system gain at approximately 1% of the nominal light output is approximately equal to 5% The open-loop system gain at the nominal light output of , and the ballast output impedance is greater than the absolute value of the maximum negative incremental lamp impedance.

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